*Why is the bottom of the lighthouse hidden by at least 24 feet on a flat earth when it should be hidden by 0 feet?*

You are trolling the upper forums.

You'd need a very powerful camera, perhaps one that needs to be invented yet, to capture the entire visual obstacle.

The video presents every aspect explicitly: no such features could be seen on a round earth.

Using metabunk calculator:

https://www.metabunk.org/curve/10.4 miles distance,

6 feet tall camera.

Gives us (really same results, but the interpretation):

With the refraction approximation* giving an effective radius of 7432.83 km (7432833.33 m)

Refracted Horizon = 5.21 km

Refracted Drop= 18.84 meters

Refracted Hidden= 8.93 meters

Refracted Horizon Dip = 0.040 Degrees, (0.0007 Radians)

Note: Not accurate for observations over water very close to the horizon (unless the temperature and vertical temperature gradient are accurate)

Geometric results (no refraction)

Geometric Horizon = 4.83 km

Geometric Drop = 21.99 meters

Geometric Hidden= 11.13 meters

Geometric Horizon Dip = 0.043 Degrees, (0.0008 Radians)

So, let's go to google earth, shall we?

I added a layer, first, 9m above the sea level:

Looks familiar?

Let's see the result on the youtube:

you can do very same without taking refraction into consideration (so his 11.4m - aka 36.5 feet). But it won't be much different.